“This forms part of our long-term plan to ensure Britain remains competitive and to secure an economic recovery for hardworking people.” Unite officials intervened last month after Ineos bosses announced that the loss-making Grangemouth petrochemical plant was closing.

Workers were ultimately forced to accept cost-cutting measures they had previously rejected in order to keep the plant open.

David Cameron has called on Labour to investigate reports of alleged intimidation tactics by unions [PA]

As part of the survival plan to save 800 jobs, employees have agreed not to strike for the next three years, move to a new pension plan and accept a three-year pay freeze.

The Prime Minister, on tour in Dubai, called on Labour to investigate reports that union members were sent to intimidate Ineos managers and their families during the industrial dispute.

Unite members were allegedly dispatched by the “leverage team” to bully plant bosses.

They were said to have told one director’s neighbours that he was “evil” and put wanted posters through the front door of the daughter of another manager.

Mr Cameron said: “If these things have happened it is serious. We shouldn’t forget that the union action at Grangemouth was in danger of killing off an absolutely vital industry for Britain and for the United Kingdom.”

Business Secretary Vince Cable said: “There are rogue unions but there are also rogue employers, some of whom have engaged in illegal tactics like blacklisting. This government will tolerate neither.”